[The impact factor: a critical analysis].

The impact factor, provided by the Institute for Scientific Information, Philadelphia (PA), has become the most important evaluation tool for scientific research and academic work. It is calculated by dividing the number of current year citations to the source items published in the journal during the previous two years. In market research, the impact factor provides quantitative evidence for editors and publishers for positioning their journals in relation to the competition. Despite its popularity, the parameter should be used with careful attention to the many phenomena that influence citation rates. The correlation between the citation frequency of a certain article and the impact factor of the journal in which it is published is questionable. A few articles have many citations and the rest are sparsely cited or not at all. Citation impact is more a measure of utility than of scientific value. Authors' selection of references is subject to biases unrelated to quality. Moreover, there is a tremendous bias towards English language journals compared with those in other languages. Finally, different specialties exhibit different ranges of peak impact. The impact factor favours research areas that promote many short-term studies. Conversely, a tendency to treat clinical investigations as less important is created.